A polyurethane having a urethane bond is manufactured mainly by a reaction of a bi- or higher functional isocyanate and a bi- or higher functional alcohol, is a polymer which excels in tensile strength, abrasion resistance, and oil resistance, and is used in a wide range of fields such as flexible foam, rigid foam, elastomer, an adhesive, a coating material, and a binder. Among them, a polyurethane made mainly from a chain or cyclic aliphatic isocyanate excels in weatherability and light resistance, and is used for fields requiring quality of appearance, such as baking coating, an automotive clear coating material, and coil coating material.
As the isocyanate, there is a case where a diisocyanate that is a bifunctional isocyanate is used, or there is a case where, for the purpose of improving physical properties and suppressing vapor pressure of a polyurethane to secure safety of workers, a diisocyanate is polymerized by reactions represented by formulas (a) to (c) to be used as an isocyanate polymer.

In the formulas, R represents a divalent organic group, and R′ represents a trivalent organic group.
An isocyanurate type isocyanate polymer is obtained in the reaction represented by formula (a), a biuret type isocyanate polymer is obtained in the reaction represented by formula (b), and a urethane type isocyanate polymer is obtained in the reaction represented by formula (c).
The biuret type isocyanate polymer is disclosed in Patent Literatures 1 to 6. The isocyanurate type isocyanate polymer is disclosed in Patent Literatures 7 to 10. An allophanate type isocyanate polymer is disclosed in Patent Literatures 11 and 12.
As described above, when being used for fields requiring quality of appearance, a polyurethane is required to be little colored. Therefore, it is important that not only coloration does not occur in a polyurethane-forming reaction but also an isocyanate (bi- or higher functional polyisocyanate) as a raw material is little colored. However, generally, an isocyanate tends to be oxidized by oxygen or the like in the air to be easily altered or colored. Moreover, when manufacturing an isocyanate polymer by polymerization of a diisocyanate, coloration of an isocyanate tends to easily occur due to a catalyst or a solvent used in the polymerization reaction.
Examples of a method for suppressing coloration of an isocyanate include a manufacturing and storing method by sealing with nitrogen gas for air shutoff, and a storing method by adding an ultraviolet absorbing agent, an antioxidizing agent and the like. For example, in order to manufacture a polyisocyanate for light-colored polyurethane lacquer, a treatment method with a peroxide after denaturation of an isocyanate is disclosed in Patent Literature 13. Moreover, a method for manufacturing an isocyanate whose coloration is reduced by contacting a colored isocyanate with ozone-containing gas is studied in Patent Literature 14. Furthermore, a method for manufacturing an isocyanate whose coloration is reduced by irradiating a colored isocyanate with light having a wavelength of 200 to 600 nm is also studied in Patent Literature 15.